See this recently asked question: Why can't I see a preview of a Word doc in the preview pane using Windows Explorer? Win 7 64bit (Always first search for similar issues addressed in the past).
However, I just tested this issue on Windows 7 Enterprise, Service Pack 1, without LibreOffice installed (work machine). If I select a LibreOffice spreadsheet (.ods) or a text document (.odt) in explorer, a preview for each is available in the preview pane (Organize >> Layout >> Preview Pane). Note that you do not get a preview as the icon in the main window - not for any file types.
The files have to have the correct filename extension - upon download my Windows initially gave them .zip extensions. You said your file was a calc file, but that it had an .odt extension. Perhaps your problem is the wrong filename extension for the file type, because this will stop it working.
Well, you learn something new every day. After seeing your further comment, I checked a few other workstations, and found variations on this issue, including one that displays thumbnails for Powerpoint files only and just icons for everything else. Curious, I googled the issue and found this: http://social.technet.microsoft.com/Forums/windows/en-US/b4a1819e-16af-4e07-9c45-49920382ced3/no-thumbnail-preview-for-word-documents-or-folders?forum=w7itproui . Sure enough, when I saved new documents in MS Office with “save thumbnail” checked, I can now get thumbnails in the explorer details pane.
However, this still didn’t give me thumbnails of odf documents in the explorer detail pane until I actually installed LibreOffice. But after installing the latest LO 22.214.171.124 on Win 7 Enterprise 64-bit, I can now get thumbnails of odt and ods documents in the explorer details pane whether they were created in MS Office or LO.
All I can say is that there are a lot of internet hits relating to issues with windows explorer not displaying thumbnails for a variety of filetypes.